Modern Christian philosophy has inherited many of the original arguments devised by the early Scholastics, but have moved on to less silly problems - primarily around the existence of god, and less about whether an angel can move from point A to point B without passing through the intervening space or whether angels know more in the morning or the evening (no kidding - these were real areas of theological debate). Still in use are arguments put forward by Christian apologists from this era (Augustine, Anselm, Aquinas, and others). But this framework has been extended by modern religious scholars (C.S. Lewis, Alvin Plantinga, William Lane Craig, Richard Swinburne). Universities around the world (mostly from the USA, but also some in the UK) are staffed with philosophers of religion who, in the last few decades, have updated the old arguments and attempted to buttress weaknesses identified in them. They have introduced new arguments and resurrected old, out of vogue ones, altering them slightly to make them more relevant to the modern era. Starting with the desired conclusion (god exists), they work backwards to construct defenses to support it.
This is quite a different approach than the one Francis Bacon, another intellectual from later during this same era advised. His method, the prototype of the scientific method, begins with observation, as free as possible from personal biases and preconceptions, or "Idols of the Mind", as he called them (of course, a complete lack of presuppositions is not possible, and arguably even not desirable). Then one should propose hypotheses to explain those observations, eliminate those that fail to adequately account for the observations, and conduct "crucial" experiments to decide among the very best of the proposed explanations. That is not the approach the Scholastics took. Their commitment to faith before anything necessarily biased the outcome of their investigations. Saint Anselm's motto was “faith seeking understanding”, and I think that this describes the investigative approach taken by the Christian philosophers of that time, and of the modern day Christians who follow them. They begin with faith, and follow with rational justification for the faith.
Aquinas's Argument from Design (which is basically the same as Paley's "A watch requires a watchmaker") is a favorite. Also popular is the modern "Fine Tuning Argument", which only could have come into existence after science discovered the seemingly fine-tuned universal constants that are required to support intelligent life. The Kalam Cosmological Argument (an old Islamic concept repopularized by Craig in the 1970's) is also frequently cited by apologists. The Discovery Institute's fully refuted and simplistic "Irreducible Complexity" argument is an attempt to disprove the possibility of the Theory (and fact) of Evolution. Plantinga puts forth a very complex and well-developed form of Foundationalism where he establishes belief in god as a foundational "properly basic belief" that can be accepted without proof (much as we accept the existence of other minds, the external universe, and the past without deductive proof). Plantinga has also joined the fight against secularism with a response to the very powerful "problem of evil". Briefly, his response is "It is possible that God, even being omnipotent, could not create a world with free creatures who never choose evil. Furthermore, it is possible that God, even being omnibenevolent, would desire to create a world which contains evil if moral goodness requires free moral creatures."
Through their many convergent philosophical efforts, they are attempting to achieve the gold standard of epistimology - the establishment of Christianity as a "justified true belief". This is a belief that has three properties: Christians believe it, it is true, and they have good "warrant" or justification for believing it. There are many beliefs that do not achieve this high bar. For example, I might believe that the field next to me contains a herd of sheep. In fact, it turns out that these are not sheep I see, but are a pack of Bedlington terriers (that I think look like sheep). This would not be a justified true belief, because it is not true. But let's then assume that behind the terriers is a flock of sheep. So, then it is true that the field contains sheep. So, I believe there are sheep in the field, and it turns out there are. But it is not a justified belief, because I believe it for the wrong reasons (I mistook the dogs for sheep). So it still lacks the status of a justified true belief. It could only achieve that if I believe that what I saw were sheep, and they actually turned out to be sheep. Christians seeking to demonstrate the logical necessity of god are striving for this level of epistemological certainty.
The modern theistic emphasis on rational approaches to support a belief in god is, to some extent, a response to the the secular "evidentialist" objection to that belief. In short, non-believers do not think there is sufficient evidence to justify a belief in god, or what evidence is presented is ambigous and unconvincing. The secular, evidentialist argument against such a belief is:
- It is irrational to believe in god without sufficient evidence.
- There is not sufficient evidence.
- Therefore, belief in God is irrational.
Continuing their pursuit of rational justification, Christian apologists have borrowed from Western philosophers Hume and Peirce the concept of "inference to the best explanation" to put forth their Christian god as the best explanation for the universe we live in. However, they leave off the second and very important part in the process of identifying causes - they stop at making what they believe to be the best inference and never then produce strong, uncontroversial, supporting evidence for the inference. It is one thing to infer a cause - but, one must then demonstrate (through evidence) the actual cause. Otherwise, all that has been done is the creation of a hypothesis, not the discovery of the actual cause. The inference is a hypothesis that needs to be tested against reality. For example, if I hear a crash outside my window, I could infer through this technique that a car crash had occurred, or that a meteor had struck, or that a building had collapsed. The "best" explanation is the car crash (they are much more common). However, I need to go outside and find the cause before I can be sure. Further, other observers seeing the same evidence should reach the same conclusion. Apologists cannot do that with god - we are just asked to accept their inference, or the evidence they produce is controversial and not universally accepted as reliable evidence.
This emphasis on evidence is not arbitrary - it is crucial. Hume introduced an important distinction between "relations of ideas" and "matters of fact". Assertions that deal with relations of ideas lend themselves to deductive logic, and are marked by the property that if taken as true, their denial would generate a contradiction. For example, all logically true statements such as "5 + 7 = 12" and "all bachelors are unmarried" are relations of ideas. Relations of ideas are intuitively or demonstrably certain, and a denial of such a proposition implies a contradiction. So, "5 + 7 = 100" would indisputably violate the logical system of mathematics, and "bachelors have multiple wives" would violate the role that bachelors play, where the concept "bachelor" entails the the concept "unmarried". However, denials of matters of fact do not force a contradiction. Denying a statement asserting a matter of fact, such as "it is raining outside" does not generate a contradiction. It is entirely possible that "it is not raining outside". Hume wrote:
"The contrary of every matter of fact is still possible; because it can never imply a contradiction, and is conceived by the mind with the same facility and distinctness, as if ever so conformable to reality. We should in vain, therefore, attempt to demonstrate its falsehood. Were it demonstratively false, it would imply a contradiction, and could never be distinctly conceived by the mind."Christian assertions that "god exists" and "god is responsible for X, Y, or Z" are matters of fact. They assert something about conditions in the world (that god is an entity who has an effect in the world). For Hume, only statements involving relations of ideas can be deduced. Matters of fact require that we gather empirical data to demonstrate whether the matter of fact is or is not true. For example, any assertion about the weather requires that we look outside to determine if it is or is not raining. It cannot be deduced. By the same token, any statement regarding god and what he does and does not do, is a matter of fact. No amount of metaphysical speculation and logical analysis will establish this. God may exist, or god may not exist - either one is conceivable in the mind, and asserting one or the other does not create a contradition, since it is a statement of a matter of fact. Kant echoed this with his distinction between "analytic judgements" (analogous to Hume's relations of ideas) and "synthetic judgements" (analogous to Hume's matters of fact). He showed that no collection of analytic statements (for example about god's necessity) could establish a synthetic conclusion (such as god exists). Hume summarized his frustration with the slippery way that other writers would try to pass off relations of ideas as matters of fact:
"If we take into our hand any volume; of divinity or school metaphysics, for instance; let us ask, Does it contain any abstract reasoning concerning quantity or number? No. Does it contain any experimental reasoning concerning matter of fact and existence? No. Consign it then to the flames: For it can contain nothing but sophistry and illusion."Although Hume and Kant made these contributions over two centuries ago, their work still stands as relevant and applicable. No other thinkers have come after them and convincingly showed them to be in error on this point. Certainly they have been improved on, extended, and clarified. But the basic difference between logical possibility and factual reality still pertains.
Another technique apologists use it so bring up perceived weaknesses in the naturalistic and/or atheistic world view. They question the assumptions of naturalism, attempting to frame them as arbitrary and without warrant (i.e., lacking philosophical and epistemological justification). They can show that some atheists have not rigorously justified their requirement for evidence of god's existence. They question the requirements of philosophical "Evidentialism", and they point out (quite rightly) that there is no deductive "proof" that obtaining knowledge though induction is a valid epistemological source. Christian presuppositionalists attempt to demonstrate that belief in uniformity of nature, the laws of nature, and the methodologies of science already assume the existence of a Christian god, or that Christianity itself is the basis of modern science, the Renaissance, the Enlightenment, and the Scientific Revolution (strange claim, considering that the established religious authorities of the time strongly resisted the intellectual advances brought on by these movements, and even still resist claims related to evolution, age of the cosmos and of earth, consciousness, and science's attempts to unravel the mystery of life and origin of the universe). Modern Christian apologetics frequently is less about defending their faith, and more focused on attacking nonbelievers in an effort to paint them as “intellectually dishonest” for not converting. So, they have upped the philosphical ante, and in a war of philosophical attrition, many atheists finally throw up their hands and confess that they either have no passion for this level of philosophical investigation, or they become overwhelmed and exhausted with the complexity of the arguments. I admit that diving into Swinburne and Plantiga, or listening to multi-hour podcast debates on "Is Belief in God Rational?" is tiring and frequently not very interesting.
So, many modern western evangelicals who don't want to accept Christianity purely on faith, who require empirical and philosophical justification, resort to mastering the rational and very intricate apologist arguments for god. One thing to keep in mind is that none of them are able to actually, then, point to that god as an entity in the world that we can all experience directly, as we would any other object or phenomenon that is claimed to exist in or affect the physical world. Physicists may ask us to belief that quarks are the building blocks of matter, and medical doctors expect us to believe that viruses and microbes underlie disease. We can't see any of these these directly, just as we can't see god directly. but there is a very important difference for quarks, microbes, and viruses - there are numerous lines of independent scientific research that all lead to the same conclusion. These multiple lines of evidence all require and support the existence of quarks, microbes, and viruses as the causal agents for the effects that they are theorized to produce. There is no evidence that these do not exist, and other competing explanations for the phenomena that they are responsible for simple do not exist. There is no counter evidence weighing against them, and the theories in which they play a role have been proven out again and again, never failing to be supported. These invisible entities, then, exists as viable and dominant scientific theories (even though they cannot actually be seen by our instruments). Side note - with the advent of electron microscopes, we now can see viruses and microbes, but before that, they fell into the same unfortunate situation as quarks, electrons, magnetic and electric fields, etc as being theoretical entities that could not actually be experienced directly.
God, still after all the philosophical justifications put forward by Christian theologicans, can only be experienced subjectively. There is no scientific or empirical method to produce the entity behind the theoretical concept of god, and there is plenty of counter evidence against god. God, as a theorized entity, does not have the same level of empirical support as other non-visible objects which we have good reason to believe exist (such as quarks, etc). After all the sophisticated philosophizing, we are left with a god who is both a metaphysical and logical possibility, but a factual myth.